Landscaping A Slope

We had an 80 foot retaining wall installed in our backyard in November. We threw some rye seed and straw down so there would not be mud everywhere.

We have bermuda mostly in our front yard so I know we need to spread some bermuda seeds now on the ground level in the back.

Any suggestions for landscaping above the retaining wall, it really is too hard to mow the steeper parts, but I can get by with the flatter areas. There's erosion and the ground is hard red clay.

I really like flowering plants that remain green year round. My ideas have been ground cover on the slope like periwinkle or wintercreeper.

I need something that is going to grow extremely fast and cheap but is appealing to the eye. I do not like juniper.

I have started planting some trailing perennials just behind the wall like creeping phlox. I am also going to plant 5 knock out roses about 5 feet back from the wall, staggered, which was a suggestion I got from a nursery. I don't know if this is the right thing to do.

I can't decide if we should use mulch or pine straw and what would stay best on the hill. We have leyland cypress lining the top of the hill. Obviously with a lot of money we could have it landscaped but what are some cost effective ideas you have.

It is such a large space. We live in Greenville, SC. Zone 7/8 I think.